1. Field of the Invention
The present invention relates to an anti-skid apparatus for installation in a vehicle braking system to prevent the road wheels of the vehicle from locking in braking operation.
2. Description of the Prior Art
In Japanese Patent Early Publication No. 60-25834 issued on Feb. 8, 1985, there has been proposed an anti-skid apparatus which includes schematically illustrated in FIG. 3, a braking circuit 1 connecting a master cylinder M/C to a wheel brake cylinder W/C, a bypass circuit 6 connected in parallel with an intermediate portion of the braking circuit 1, a first solenoid valve Va of the normally closed type disposed within the bypass circuit 6, a second solenoid valve Vb of the normally open type disposed within the intermediate portion of the braking circuit 1, a fluid reservoir Ra disposed within the bypass circuit 6, a fluid pump P connected to the fluid reservoir Ra to produce a hydraulic power pressure therefrom, and an accumulator ACC connected to the bypass circuit 6 to store the hydraulic power pressure applied thereto from the fluid pump P through a check valve. The anti-skid apparatus further includes a check valve Vc of the normally open type disposed within the braking circuit 1, and a check valve 7 disposed within the bypass circuit 6 to interrupt the flow of fluid under pressure from the braking circuit 1 to the accumulator ACC. The check valve Vc includes a ball 2 engageable with a valve seat 3, and a coil spring 4 arranged to bias the ball 2 toward the valve seat 3. The fluid reservoir Ra includes a piston 9 loaded by a coil spring 8 toward the ball 2 of check valve Vc. Disposed between the ball 2 and piston 9 is a push rod 5 which is arranged to normally separate the ball 2 from the valve seat 3 under the load of coil spring 8.
In braking operation, the check valve Vc permits the flow of fluid under pressure supplied to the wheel brake cylinder W/C from the master cylinder M/C. When a road wheel tends to be locked in the braking operation, the first solenoid valve Va is energized to connect the wheel brake cylinder W/C to the fluid reservoir Ra through the bypass circuit 6, the second solenoid valve Vb is energized to close the intermediate portion of braking circuit 1, and the fluid pump P is driven to produce a hydraulic power pressure therefrom. As a result, the piston 9 of reservoir Ra is moved against the load of spring 8 to store the fluid under pressure supplied thereto from the wheel brake cylinder W/C. Subsequently, the check valve Vc is closed in response to retraction of the push rod 5 and maintained in its closed position under the pressure in braking circuit 1. During energization of the solenoid valves Va and Vb, the accumulator ACC acts to store the hydraulic power pressure applied thereto from the fluid pump P. When the solenoid valves Va and Vb are deenergized in response to a decrease of the wheel brake cylinder pressure, the power pressure is applied to the wheel brake cylinder W/C from the accumulator ACC through the check valve 7.
In such a conventional anti-skid apparatus as described above, the hydraulic power pressure may not be controlled in accordance with the braking pressure created in the master cylinder in braking operation. For this reason, if the road wheel tends to be locked by a low pressure in the wheel brake cylinder W/C in braking operation on a snow or frozen road, there will occur a large difference in pressure between the wheel brake cylinder W/C and the accumulator ACC. If the road wheel tends to be locked by a high pressure in the wheel brake cylinder W/C in braking operation on a normal road, there will occur a small difference in pressure between the wheel brake cylinder W/C and the accumulator ACC. This means that under control of the solenoid valves Va, Vb, the wheel brake cylinder pressure is rapidly increased in braking operation on a snow or frozen road and is slowly increased in braking operation on a normal road. It is, therefore, required to control the solenoid valves Va, Vb in a complicated manner for preventing the wheel road from locking in braking operation.
Furthermore, in the case that the road wheel tends to be locked by a low pressure in the wheel brake cylinder W/C, a small amount of the fluid under pressure is supplied to the wheel brake cylinder W/C from the accumulator ACC under control of the solenoid valves Va, Vb. In such a condition, the amount of fluid under pressure discharged from the pump P exceeds the amount of the fluid under pressure supplied to the wheel brake cylinder. This causes an excessive increase of the pressure in the accumulator ACC, resulting in an increase of the load acting on the pump P. Additionally, in the anti-skid apparatus, the piston 9 of reservoir Ra is displaced by the braking pressure applied thereto through the push rod 5 in braking operation. For this reason, if the solenoid valves Va, Vb are energized in a condition where the braking pressure is high, the reservoir piston 9 will be rapidly moved by the fluid under pressure supplied thereto from the wheel brake cylinder W/C. If the solenoid valves Va, Vb are energinzed in a condition where the braking pressure is low, the reservoir piston 9 will be slowly moved by the fluid under pressure supplied thereto from the wheel brake cylinder W/C. This causes undesired influence in operation of the anti-skid apparatus for pressure reduction in the wheel brake cylinder.